no. R2

A general theorem on triangular finite C (m) -elements
ESAIM: Mathematical Modelling and Numerical Analysis - Modélisation Mathématique et Analyse Numérique, Tome 8 (1974) no. R2, pp. 119-127. 
@article{M2AN_1974__8_2_119_0,
     author = {\v{Z}en{\'\i}\v{s}ek, Alexander},
     title = {A general theorem on triangular finite $C^{(m)}$-elements},
     journal = {ESAIM: Mathematical Modelling and Numerical Analysis - Mod\'elisation Math\'ematique et Analyse Num\'erique},
     pages = {119--127},
     publisher = {Dunod},
     address = {Paris},
     volume = {8},
     number = {R2},
     year = {1974},
     zbl = {0321.41003},
     mrnumber = {388731},
     language = {en},
     url = {http://archive.numdam.org/item/M2AN_1974__8_2_119_0/}
}
Ženíšek, Alexander. A general theorem on triangular finite $C^{(m)}$-elements. ESAIM: Mathematical Modelling and Numerical Analysis - Modélisation Mathématique et Analyse Numérique, Tome 8 (1974) no. R2, pp. 119-127. http://archive.numdam.org/item/M2AN_1974__8_2_119_0/

[1] Bell K., A refined triangular plate bending finite element Int. J. Num. Meth. Engng., 1969, 1, 101-122.

[2] Bramble J. H. and Zlámalm., Triangular elements in the finite element method. Math. Comp., 1970, 24, 809-820. | MR 282540 | Zbl 0226.65073

[3] Koukals S., Piecewise polynomial interpolations in the finite element method. Apl.Mat., 1973, 18, 146-160. | MR 321318 | Zbl 0305.65070

[4] Strang G. and Fix G., An analysis of the finite element method. Prentice-Hall Inc., Englewood Cliffs, N. J., 1973. | MR 443377 | Zbl 0356.65096

[5] Žen>Ĺšek A., Interpolation polynomials on the triangle. Numer. Math., 1970, 15, 283-296. | MR 275014 | Zbl 0216.38901

[6] Žen>Ĺšek A., Polynomial approximation on tetrahedrons in the finite element method.J. Approx. Theory, 1973, 7, 334-351. | MR 350260 | Zbl 0279.41005